Phospholipase A2 (PLA2) has been shown to be only active at lipid domain-domain boundaries or local structural defects in model membranes. We are investigating this mechanism and its potential relevance in living RBL cell membranes. Using biologically novel fluorescent membrane probes that undergo a spectral shift upon hydrolysis, we have found that antigenic stimulation of these cells enhances exogenous PLA2 activity, implying a cell-regulated lipid structural rearangement. A manuscript describing these results is currently under preparation.